Television receivers are vulnerable to noise signals from various sources. The interference usually results from electromagnetic radiation which interferes with the broadcast RF signal. Typical sources of such interference are automotive ignition systems and household motors such as washing machines, dryers and refrigerators. Similar types of interference may also be caused in a broadcast RF signal by high definition television systems and digitized 525 line systems. The interfering signals often appear as bright or dark spots on the television screen.
The patent literature describes numerous approaches to reducing or eliminating the problems caused by interference in broadcast television and/or video signals. The general approach to the problem has been to design circuits that combat the effects of interfering signals, and then place these circuits in the signal path of the incoming television/video signal. Unfortunately, few circuit designs deal effectively with more than one type of interference. Accordingly, if it is desired to combat four types of interference, the television receiver will typically have four interference compensation circuits placed in its incoming signal path. Each of the four circuits process the incoming signal regardless of whether interference is present, and regardless of whether the interference is the type that the particular circuit is designed to respond to. Also, such schemes may actually cause interference because a given circuit could react unpredictably when exposed to a type of interference that it was not designed to combat, or when trying to correct problems that have already been acted on by another circuit.
Because television pictures are formed by scanning an image and partitioning that image into a plurality of horizontal lines, the signal information from image line to image line is highly redundant for a significant percentage of images. The horizontal lines are sequentially transmitted to the television's receiver circuitry where they are reconstructed, line by line, into the original image and shown on the television screen.
U.S. Pat. No. 2,996,576, issued to Dolby, discloses a system which takes advantage of the line to line signal redundancy inherent in scanning and partitioning images, and minimizes the effect of television signal disturbances in video recording/reproducing systems such as video recorders and video disk systems. Video signals are typically recorded as a frequency modulation (FM) of a carrier. The Dolby system responds to disturbing signals that have sufficient amplitude to cause loss of the FM carrier. Thus, video signal loss may be detected by detecting the loss of the FM carrier. This is commonly referred to as "envelope detection." Upon detection of a video signal loss, the signal for the preceding image line, which is delayed by one image line period, is substituted for the lost signal to minimize picture disturbance.
The relatively large number of defects in video recording media makes it imperative to include signal compensation systems in video recording/reproducing apparatus. U.S. Pat. No. 5,032,915, issued to Ichimura, discloses such a noise detection and dropout sensing and compensation circuit for use in video recorders.
By contrast, it was, for a period of time, too costly to include such interference correction systems in consumer television receivers. However, with the incorporation of charge transfer device delay lines in television receivers to perform comb filtering of the luminance signal, it has become practical in consumer televisions to consider correcting interference signals such as, for example, impulse noise. The interference correction can be performed by video signal substitution as in video recording/reproducing systems. However, the detection of noise or defects in broadcast television signals presents different problems than those encountered in video recording/reproducing systems.
One such difference occurs where the interference or noise creating the picture disturbance does not have sufficient amplitude, as would an impulse-type noise, to eliminate the broadcast signal carrier. In those cases, one cannot rely upon envelope detection of the broadcast signal carrier to determine when defects are present in the signal. In addition, television signals are broadcast as an amplitude modulated carrier so that amplitude changes are generally not indicative of the presence of noise or signal defects. In other words, because the dynamic amplitude range of the baseband video signal is very large, noise can have amplitudes ranging from barely perceptible to large enough to overload the receiver circuit.
U.S. Pat. No. 4,353,093, issued to Durbin, recognized that impulse noise, i.e., a noise signal that may supersede the video signal on all or a part of an image line, typically has a broad energy spectrum and causes interference across many television channels. Therefore, energy from the impulse is likely to be included in the sound component of a particular television signal even though the sound component is broadcast on a carrier separate from the video signal component of the particular television signal. Because the sound component is a high frequency modulated wave form in the baseband TV signal, amplitude detection of such a signal can be used for effectively detecting the presence of relatively small values of narrow duration/wider bandwidth noise.
In the Durbin patent, a phase-locked loop (PLL) synchronous detector arrangement is responsive to a bandpass filtered version of the modulated sound carrier for detecting amplitude variations of the sound carrier. Compensation circuitry, including a delay line, coupled to the output of the synchronous detector substitutes a prior image line for the present image line in response to the level of the detected amplitude variations. In addition to this PLL synchronous detector arrangement for detecting impulse noise, conventional intercarrier detection circuitry is required for detecting the audio and video information from the composite television signal.
U.S. Pat. No. 4,514,763, issued to Rindal, takes the same basic approach as the Durbin patent to eliminating impulse noise. The Rindal patent, however, uses a PLL for detecting the audio information and providing a defect control signal for reducing impulse noise. One drawback of using audio high frequencies (which are not in the video channel) is that the sound channel could be affected when the video is not, or vice versa. Thus, the reliability of such systems is compromised.
Thus, currently available television signal noise reduction systems do not provide the user with sufficient control over how and when the individual noise reduction circuits are used. Accordingly, the available systems do not counteract the potentially unpredictable reaction of a noise reduction circuit when it receives a type of noise or other signals that it was not designed to process.. Also, the individual circuits available for combating interfering signals such as impulse noise, for example, do not provide sufficient reliability.